A good outline can save you hundreds of hours of effort that would otherwise be spent rewriting. Knowing exactly how the individual pieces of your book fit together can give you clarity and direction, strengthening the final result.
Plot and structure problems should be resolved in the outlining phase, before the writing begins. Then you'll know exactly what to write about each time you work on your book, and after each successful session you can begin thinking about the next.
A non-fiction book is nothing more than a series of essays. A fiction book is nothing more than a series of scenes. The art of structure is found in learning to place them in logical order.
Your first outlines might be somewhat generic, but they'll get more detailed and useful with practice. For me, creating a good outline is the hardest part of writing a book, but it pays continual dividends and is ultimately well worth the effort. Houses begin with blueprints, and good books begin with good outlines.